Rod-pumping system

ABSTRACT

A rod-pumping system is provided. In one embodiment, the system includes a downhole pump positioned in a well and coupled to a well string, such as a sucker-rod string. The system also includes a first hydraulic actuator arranged with respect to the well string so as to enable the first hydraulic actuator to move the well string within the well. The first hydraulic actuator is connected in fluid communication with a second hydraulic actuator. A control pump is connected to both the first and second hydraulic actuators to enable the control pump to alternate between pumping control fluid to the first hydraulic actuator to cause the well string to move in a first direction within the well and pumping control fluid to the second hydraulic actuator to cause the well string to move in an opposite, second direction within the well. Additional systems, devices, and methods are also disclosed.

BACKGROUND

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the presently describedembodiments. This discussion is believed to be helpful in providing thereader with background information to facilitate a better understandingof the various aspects of the present embodiments. Accordingly, itshould be understood that these statements are to be read in this light,and not as admissions of prior art.

In order to meet consumer and industrial demand for natural resources,companies often invest significant amounts of time and money in findingand extracting oil, natural gas, and other subterranean resources fromthe earth. Particularly, once a desired subterranean resource such asoil or natural gas is discovered, drilling and production systems areoften employed to access and extract the resource. These systems may belocated onshore or offshore depending on the location of a desiredresource. Further, such systems generally include a wellhead assemblymounted on a well through which the resource is accessed or extracted.These wellhead assemblies may include a wide variety of components, suchas various casings, valves, pumps, fluid conduits, and the like, thatcontrol drilling or extraction operations.

In some instances, resources accessed via wells are able to flow to thesurface by themselves. This is typically the case with gas wells, as theaccessed gas has a lower density than air. This can also be the case foroil wells if the pressure of the oil is sufficiently high to overcomegravity. But often accessed oil does not have sufficient pressure toflow to the surface and the oil must be lifted to the surface throughone of various methods known as artificial lift. Artificial lift canalso be used to raise other resources through wells to the surface, orfor removing water or other liquids from gas wells. One form ofartificial lift uses a pump that is placed downhole in the well and isoperated by a reciprocating rod string extending through the well fromthe downhole pump to the surface. Such systems are commonly referred toas rod-pumping or sucker-rod systems.

SUMMARY

Certain aspects of some embodiments disclosed herein are set forthbelow. It should be understood that these aspects are presented merelyto provide the reader with a brief summary of certain forms theinvention might take and that these aspects are not intended to limitthe scope of the invention. Indeed, the invention may encompass avariety of aspects that may not be set forth below.

Embodiments of the present disclosure generally relate to a rod-pumpingsystem for lifting fluids from a well. The rod-pumping system of oneembodiment includes a pair of hydraulic actuators, such as hydrauliccylinders; a rod string moved by one of the hydraulic actuators andcoupled to a downhole pump; and a control pump. The hydraulic actuatorsare connected in series, and the control pump is connected to pump fluidto one end of each hydraulic actuator such that alternating the flowdirection from the control pump controls operation of the hydraulicactuators and operates the downhole pump via the rod string. In someembodiments, sensors are used to detect the positions of pistons in theactuators and a controller synchronizes the pistons to facilitate properoperation.

Various refinements of the features noted above may exist in relation tovarious aspects of the present embodiments. Further features may also beincorporated in these various aspects as well. These refinements andadditional features may exist individually or in any combination. Forinstance, various features discussed below in relation to one or more ofthe illustrated embodiments may be incorporated into any of theabove-described aspects of the present disclosure alone or in anycombination. Again, the brief summary presented above is intended onlyto familiarize the reader with certain aspects and contexts of someembodiments without limitation to the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of certain embodimentswill become better understood when the following detailed description isread with reference to the accompanying drawings in which likecharacters represent like parts throughout the drawings, wherein:

FIG. 1 generally depicts a production system having an artificial liftsystem to draw fluid from a well to the surface in accordance with oneembodiment of the present disclosure;

FIG. 2 is a block diagram of various components of the artificial liftsystem of FIG. 1 in accordance with one embodiment;

FIG. 3 generally depicts a hydraulic actuator of the artificial liftsystem of FIG. 2 mounted on wellhead equipment in accordance with oneembodiment;

FIG. 4 is a cross-section generally depicting certain components of thehydraulic actuator of FIG. 3 in accordance with one embodiment;

FIG. 5 generally depicts the hydraulic actuator of FIG. 3 with a pistonrod of the actuator extended from its housing; and

FIG. 6 schematically depicts certain hydraulic components and controlcomponents of the production system of FIG. 1 in accordance with oneembodiment of the present disclosure.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present disclosure will bedescribed below. In an effort to provide a concise description of theseembodiments, all features of an actual implementation may not bedescribed in the specification. It should be appreciated that in thedevelopment of any such actual implementation, as in any engineering ordesign project, numerous implementation-specific decisions must be madeto achieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

When introducing elements of various embodiments, the articles “a,”“an,” “the,” and “said” are intended to mean that there are one or moreof the elements. The terms “comprising,” “including,” and “having” areintended to be inclusive and mean that there may be additional elementsother than the listed elements. Moreover, any use of “top,” “bottom,”“above,” “below,” other directional terms, and variations of these termsis made for convenience, but does not require any particular orientationof the components.

Turning now to the present figures, a system 10 is illustrated in FIG. 1in accordance with one embodiment. Notably, the system 10 is aproduction system that facilitates extraction of a resource, such asoil, from a reservoir 12 through a well 14. Wellhead equipment 16 isinstalled on the well (e.g., attached to the top of casing and tubingstrings in the well). In one embodiment, the wellhead equipment 16includes a casing head, a tubing head, and a stuffing box. But thecomponents of the wellhead equipment 16 can differ between differentapplications, and such equipment could include various casing heads,tubing heads, stuffing boxes, pumping tees, and pressure gauges, to nameonly a few possibilities.

The system 10 also includes an artificial lift system 18. In oneembodiment generally depicted in FIG. 2, the artificial lift system 18is a rod-pumping system including a sucker-rod string 22 coupled to adownhole pump 24 in the well 14. The downhole pump 24 can be of anysuitable design, such as one in which the downhole pump 24 includes astationary valve and a traveling valve (connected to the sucker-rodstring 22) that cooperate to lift fluid from a reservoir to the surface.The sucker-rod string 22 extends through the well 14 and is also coupledto a linear actuator, such as slave cylinder 26 in FIG. 2. This linearactuator enables movement of the sucker-rod string 22 back-and-forthwithin the well 14 to operate the downhole pump 24 and raise fluid(e.g., oil, gas condensate, or water) from the reservoir 12 to thesurface. In other embodiments, the sucker-rod string 22 could bereplaced by some other structure (e.g., a coiled tubing string)connected to the downhole pump 24 and moved by the linear actuator tolift fluids from the reservoir 12. As used herein, the term “wellstring” means any device or structure that extends through a well andenables a linear actuator (e.g., slave cylinder 26) to operate adownhole pump. The term encompasses, but is not limited to, bothsucker-rod string and coiled tubing.

As depicted in FIG. 2, the artificial lift system 18 includes anadditional linear actuator in the form of master cylinder 28. The mastercylinder 28 cooperates with the slave cylinder 26 in controllingmovement of the sucker-rod string 22, as discussed in greater detailbelow. Although here described as cylinders 26 and 28, it will beappreciated that the linear actuators could be provided in any suitableform (including non-cylindrical shapes, for instance) and may also bereferred to as rams, jacks, or the like. In some embodiments, theartificial lift system 18 is a hydraulic system with hydraulic linearactuators. But while the examples provided below refer to such ahydraulic system, it is noted that in other embodiments the artificiallift system 18 can instead be a pneumatic system with pneumatic linearactuators that are operated with a gaseous control fluid, such ascompressed air.

The artificial lift system 18 depicted in FIG. 2 also includes a pumpingsystem 30. This pumping system 30 includes a primary pump 32 and anauxiliary pump 34. As will be described in greater detail below, theprimary pump 32 is connected in fluid communication with both of thehydraulic actuators (i.e., slave cylinder 26 and master cylinder 28 inthe depicted embodiment) to control movement of pistons inside theseactuators. Thus, primary pump 32 is also referred to herein as controlpump 32. In one embodiment, the control pump 32 is a bidirectional pumpcapable of pumping hydraulic control fluid (e.g., hydraulic oil)back-and-forth between the slave cylinder 26 and the master cylinder 28by reversing the flow direction. If additional control fluid is needed,the control pump 32 can draw such fluid from a fluid tank 38. Thecontrol pump 32 is driven by a prime mover 36, such as a diesel engine.But any suitable prime mover 36 could be used, such as a propane engine,a natural gas engine (which could include an engine run on casing headgas produced at the well 14), or an electric motor.

The auxiliary pump 34 is connected to provide fresh control fluid (i.e.,new or reconditioned control fluid) to at least some portions of thehydraulic circuit that includes the slave cylinder 26 and the mastercylinder 28. The auxiliary pump 34 can also, but need not, draw controlfluid from the fluid tank 38 and be driven by the prime mover 36independent of the control pump 32. (It is noted that taking a slipstream off of the control pump circuit to feed the auxiliary pump 34,while possible, would reduce system speed.) In one embodiment, controlfluid is flushed from various portions of the hydraulic circuit (e.g.,with one or more flushing valves) and replaced by fresh control fluidpumped from the fluid tank 38 by the control pump 32 and the auxiliarypump 34. The flushed control fluid can be routed through a conditioningsystem 40, such as through one or more filters to remove anycontaminants in the control fluid and through a cooler that lowers thetemperature of the control fluid to reduce wear on components of theartificial lift system 18. Control fluid reconditioned by theconditioning system 40 can be returned to the fluid tank 38 to be reusedin the pumping system 30.

The pumping system 30 also includes a controller 42. The controller 42processes inputs from various sensors 44 to control operation of othercomponents of the pumping system 30 (e.g., the control pump 32, theauxiliary pump 34, the prime mover 36, any flushing valves, and thecooler of the conditioning system 40) and, by extension, operation ofthe slave cylinder 26 and the master cylinder 28. In some embodiments,the sensors 44 include proximity switches and a linear positiontransducer that allow the determination of positions of pistons withinthe slave cylinder 26 and the master cylinder 28. The sensors couldalso, for example, include a temperature sensor to monitor temperaturesof control fluid or components in the artificial lift system 18,pressure sensors to detect hydraulic pressures at various locationswithin the system 18, or a level sensor to detect the amount of controlfluid available in the fluid tank 38. The controller 42 in oneembodiment is a programmable logic controller that is programmed toprovide the control functionality described herein. But in otherembodiments, the controller 42 could be any circuit-based device (withor without software) suitable for controlling operation of theartificial lift system 18, such as a processor-based device thatexecutes instructions (firmware or software) stored in a suitable memoryof the device.

The depicted pumping system 30 also includes a skid 46 to facilitatetransport of various system components. For example, in one embodimentthe primary pump 32, the auxiliary pump 34, the prime mover 36, thefluid tank 38, the conditioning system 40, the controller 42, and somesensors 44 are provided on the skid 46, allowing an operator to moreeasily move all of these components to a desired location. Othercomponents, such as the master cylinder 28, can also be mounted on theskid 46.

As noted above, the slave cylinder 26 engages the sucker-rod string 22to operate the downhole pump 24 and lift fluid up the well 14 to thesurface. The slave cylinder 26 can be attached to wellhead equipment toreceive the sucker-rod string 22 and operate the downhole pump 24. Oneexample of such an arrangement is depicted in FIGS. 3-5. In thisembodiment, the slave cylinder 26 is positioned over a polished rod 50of the sucker-rod string 22 and is connected, via a pedestal stand 52,to a tubing head 54 of the wellhead equipment 16. Although not depictedin the present figure, the tubing head 54 can be coupled to otherwellhead equipment 16, such as a casing head, above the well 14. Thetubing head 54 includes a flow conduit 56 for conveying fluid liftedfrom the well 14, as well as a test conduit 58. While the slave cylinder26 is depicted as being connected to the tubing head 54 by the pedestalstand 52 in FIG. 3, the slave cylinder 26 can be connected to thewellhead equipment 16 in any suitable manner (e.g., connected in otherways or to other components of the wellhead equipment 16). Indeed, insome embodiments the slave cylinder 26 is not connected to the wellheadequipment 16 at all, and is instead positioned adjacent the wellheadequipment 16. In one example of such an embodiment, a slave cylinder 26is mounted adjacent to, but separated from, the wellhead equipment 16and interacts with the sucker-rod string 22 via cables and a bridle.

The wellhead equipment 16 also includes a stuffing box 60 attached tothe tubing head 54. As will be recognized by those knowledgeable in theart, the stuffing box 60 includes packing that allows the polished rod50 to move up-and-down through the stuffing box 60 while inhibitingleaking. The polished rod 50 is connected to a series of sucker rods toform the sucker-rod string 22 that extends through the well 14 to thedownhole pump 24. Movement of the polished rod 50 causes correspondingmovement of the sucker rods to operate the downhole pump 24. In otherembodiments, the stuffing box 60 is omitted and the slave cylinder 26itself isolates wellbore fluids from the external environment.

The slave cylinder 26 includes a housing 66 in which a piston rod 68 isdisposed. The piston rod 68 of the depicted embodiment is hollow (seeFIG. 4) and a connecting rod 70 is disposed within the bore of thepiston rod 68. The connecting rod 70 is coupled to the polished rod 50as part of the sucker-rod string 22. A rod clamp 72 is coupled to theconnecting rod 70 and is positioned at the end of the piston rod 68 toallow the sucker-rod string 22 to be suspended in the well 14. In otherembodiments, the connecting rod 70 is omitted and the polished rod 50 ofthe sucker-rod string 22 engages the slave cylinder 26 (e.g., via a rodclamp 72 attached to the polished rod 50 itself). By applyingappropriate pressure, the piston rod 68 can be extended or retractedwith respect to the housing 66. For instance, in the present embodimenthydraulic control fluid can be provided into the housing 66 through aconnection 78 at the cap end of slave cylinder 26 to cause the pistonrod 68 to extend from the housing 66, and through a connection 80 at therod end of the slave cylinder 26 to cause the piston rod 68 to retractinto the housing 66. For convenience, the slave cylinder 26 can includea fluid conduit 82 and a connection 84 coupled to the connection 80,allowing a fluid supply hose or pipe to be connected to themore-accessible (i.e., closer to the ground) connection 84. Although notdepicted in the present figure, in one embodiment the primary pump 32 isconnected directly to the connection 84 via a hose or pipe and themaster cylinder 28 is connected to the slave cylinder 26 via theconnection 78. Also, the rod clamp 72 in some embodiments rests on thepiston rod 68 such that the sucker-rod string 22 is lifted by extensionof the piston rod 68 from the housing 66 but is lowered by gravity whenthe piston rod 68 retracts. In other embodiments, the rod clamp 72 canbe attached to the piston rod 68 such that the sucker-rod string 22 isdriven by the piston rod 68 in both directions. In either of theseinstances, movement of the piston rod 68 can be said to cause reciprocalmovement of the sucker-rod string 22.

As depicted in FIG. 4, the hollow piston rod 68 may be mounted about ahollow tube 90 in the housing 66. The tube 90 provides a bearing surfacefor the hollow piston rod 68 and isolates the working pressures in theslave cylinder 26 from the connecting rod 70 and other components in thewell 14. The piston rod 68 may be extended or retracted by manipulatingpressure against the piston head 92. In the embodiment depicted in FIGS.3-5, the slave cylinder 26 is a double-acting cylinder. Specifically,pressurized hydraulic control fluid can be directed into the chamber 96(e.g., via connection 78) to cause the piston head 92 to move and thepiston rod 68 to extend from the housing 66, as depicted in FIG. 5.Conversely, such control fluid can be directed into the chamber 98(e.g., via connection 80) to move the piston head 92 in an oppositedirection and to retract the piston rod 68 to the position depicted inFIG. 3. Although not depicted in the present figures for the sake ofclarity, it will be appreciated that the slave cylinder 26 willgenerally include additional components, such as various seals thatinhibit leakage from the housing 66 or between the chambers 96 and 98.

As noted above, the connecting rod 70 is positioned with respect to thepiston rod 68 and is coupled to the sucker-rod string 22 to enable themovement of the piston rod 68 to operate the downhole pump 24. In theembodiment depicted in FIG. 4, the connecting rod 70 is coupled to thepolished rod 50 of the sucker-rod string 22 with a connector 102. Butthe connecting rod 70 (or, in the absence of the connecting rod 70, theslave cylinder 26) could engage the polished rod 50 in any suitablemanner that allows the slave cylinder 26 to operate the downhole pump24.

In at least some embodiments, the slave cylinder 26 includes one or moresensors to detect the position of the piston head 92 within the housing66. Any suitable sensor could be used, such as a proximity switch or alinear transducer. In one embodiment depicted in FIGS. 3 and 5, sensorsare provided on an external guide 108 and include proximity sensors orswitches 110 and 112. As one example, the external guide 108 can be alength of pipe (e.g., PVC pipe) with proximity switches 110 and 112mounted on its exterior. A trigger 114, such as a piece of metal in thecase of inductive proximity switches, is connected to a support 116. Thesupport 116 moves with the piston rod 68, causing the trigger 114 tomove between the proximity switches 110 and 112. As the piston rod 68extends from the housing 66 toward the position depicted in FIG. 5, thetrigger 114 is drawn upwardly through the guide 108 toward proximityswitch 110. Once the proximity switch 110 is activated by the trigger114, an output signal is sent to the controller 42 to cause the pistonrod 68 to begin to retract back into the housing 66 (toward the positiondepicted in FIG. 3). While the piston rod 68 retracts, the trigger 114travels through the guide 108 toward the proximity switch 112. Once thisswitch 112 is activated, an output signal is sent to the controller 42to cause the piston rod 68 to reverse direction again. In this manner,the proximity switches 110 and 112 facilitate reciprocal motion of thepiston rod 68 between the extended and retracted positions and of thesucker-rod string 22 within the well 14. In another embodiment, theposition of the master cylinder 28 (e.g., as determined by a linearposition transducer 138 (FIG. 6)) is used to control operation of thesystem and output from the switches 110 and 112 can be compared to thedetected position of the master cylinder 28 to verify proper operationof the system.

Certain operational aspects of the rod-pumping system described abovemay be better understood with reference to FIG. 6, which is a schematicdiagram generally depicting hydraulic connections between certainhydraulic components of the system 18 and control of these hydrauliccomponents by the controller 42 in accordance with one embodiment. Inthis diagram, the master cylinder 28 includes a floating piston 120having a piston rod between piston heads 122 and 124. As noted abovewith respect to the slave cylinder 26, it will be appreciated that themaster cylinder 28 will generally include other components that are notdepicted in the present figure, including seals that inhibit leakingfrom, or between different portions of the cylinder 28. The piston rodof piston 120 is positioned through a wall or bulkhead 126 of the mastercylinder 28 with the piston heads 122 and 124 disposed on opposite sidesof the bulkhead. This generally divides the interior of the mastercylinder 28 into four isolated chambers, referred to herein as chambers128, 130, 132, and 134.

The master cylinder 28 is arranged in series with the slave cylinder 26,with the chamber 128 connected in fluid communication with the chamber96 on the cap end of the slave cylinder 26. Primary pump 32 is connectedin fluid communication with both the chamber 130 and the chamber 98 onthe rod end of the slave cylinder 26. Pressurized hydraulic fluid in thechambers 128 and 130 may be manipulated to act on the piston head 122and move the piston 120 within the master cylinder 28. The mastercylinder 28 in FIG. 6 is assisted by one or more accumulators 136connected to the chamber 134. As will be appreciated, the accumulators136 are energy storage devices that apply pressure to hydraulic fluid inthe accumulators. Any suitable accumulators 136 may be used, but in atleast some embodiments the accumulators 136 are compressed gasaccumulators (e.g., nitrogen accumulators). Pressure stored in theaccumulators 136 is transmitted via hydraulic fluid in the chamber 134to the piston head 124. Chamber 132 in FIG. 6 is connected in fluidcommunication with the fluid tank 38 so as to not inhibit movement ofthe piston 120 in response to hydraulic pressures within chambers 128,130, and 134. Specifically, chamber 132 is connected to draw fluid fromthe fluid tank 38 when chamber 132 expands and to expel the fluid backto the fluid tank 38 when the chamber 132 contracts.

The system depicted in FIG. 6 is a proportional, intelligent,closed-loop hydraulic system in which feedback from various sensors(e.g., proximity switches 110 and 112, a position transducer 138, andother sensors 144) is used by the controller 42 to adjust operation ofthe system. During operation, primary pump 32 alternates pumping ofpressurized control fluid between the chamber 130 of the master cylinder28 and chamber 98 at the rod end of the slave cylinder 26. Particularly,pumping of the control fluid into the chamber 130 by the primary pump 32causes the piston 120 to move to the left in FIG. 6, and movement inthis direction is assisted by the pressure stored in the accumulators136 (transmitted via fluid in chamber 134). Movement of the piston 120in this manner reduces the volume of chamber 128, causing pressurizedcontrol fluid to flow from chamber 128 toward chamber 96 at the cap endof the slave cylinder 26. This in turn causes the piston head 92 to movetoward the rod end of the slave cylinder 26 and extends the piston rod68 from the housing 66 of the slave cylinder 26 to raise the sucker-rodstring 22 in the well. Such motion can be generally referred to as anupstroke. Conversely, the primary pump 32 can pump fluid into thechamber 98 of the rod end of the slave cylinder 26 to move the pistonhead 92 away from the rod end and retract the piston rod 68. Thismovement causes the sucker-rod string to be lowered further into thewell and may be referred to as a downstroke. The movement of the pistonhead 92 during the downstroke causes control fluid to flow from thechamber 96 toward the chamber 128, causing the master cylinder piston120 to move to the right in FIG. 6. This also causes pressurized fluidfrom chamber 134 to be pushed into the accumulators 136, thereby storingenergy for the next upstroke.

In one embodiment, the one or more accumulators 136 are set (e.g.,pre-charged) to counterbalance the full load of the slave cylinder 26during operation. That is, the force on the master cylinder piston 120caused by the one or more accumulators 136 meets or exceeds the load onthe slave cylinder piston due to gravity (which includes loading by thecomponents borne by the piston rod 68, such as the sucker-rod string 22and any portion of the downhole pump 24 connected to move with thesucker-rod string 22). This fully counterbalanced arrangement is incontrast to systems in which the load on the slave cylinder is onlypartially counterbalanced. In such partially counterbalanced systems,upstrokes rely on pressure from control pumps to provide sufficientforce to overcome gravitational loading on pistons of slave cylinderscoupled to a rod string and the systems rely on gravity to retract theslave cylinder piston and push the master cylinder piston back towardaccumulators. But having the primary pump 32 control one side of theslave cylinder 26, as described above, reduces or eliminates thereliance on gravity to retract the slave cylinder piston, allowing thecounterbalance pressure from the accumulators 136 to be set at or abovethe load on the slave cylinder piston.

In one embodiment, the biasing force from the accumulators 136 balancesthe full load on the piston of slave cylinder 26, and the primary pump32 is used to offset this hydraulic balance between the cylinders toprovide directional control and speed control of the piston rod 68 (and,by extension, of the sucker-rod string 22). As one line from the primarypump 32 is connected to the rod end of the slave cylinder 26 (ratherthan the pump having direct control over the reciprocating of the mastercylinder 28 by being directly connected to both sides of its piston),the pistons of the two cylinders move in consort based on differentialpressures. Such an arrangement allows the use of a differential cylinder(e.g., slave cylinder 26) in a closed-loop system without venting toatmosphere. This is in contrast to other arrangements that rely ongravity to reset the piston of a slave cylinder and in which the rod endof the slave cylinder is vented to atmosphere so as to not inhibitmovement of the piston. By not venting the rod end of the slave cylinder26 to atmosphere, the present arrangement avoids large pressuredifferentials across seals provided on piston head 92 to isolate thechambers 96 and 98 (such pressure differentials could contribute topremature seal failure) and reduces the likelihood of rust andcontamination of the rod end components of the slave cylinder 26, likepiston rod 68.

In at least some embodiments, the master cylinder 28 is constructedproportionally to the slave cylinder 26 to have larger piston heads anda shorter stroke. The connection of the primary pump 32 to both theslave cylinder 26 and the master cylinder 28 in the manner depicted inFIG. 6 also allows the cylinder pistons to be controlled by applyingpressure to less surface area on the piston heads of the cylinderpistons. Particularly, the piston rod 68 may be retracted and the mastercylinder piston 120 pushed toward the accumulators by pumping fluid intochamber 98 to act on the effective area of the piston head 92 about thepiston rod 68 rather than pumping fluid into chamber 128 to act on thegreater surface area of the piston head 122. By controlling operationvia a smaller piston head area, the present arrangement increases systemefficiency and allows the use of a smaller control pump 32, smaller flowfrom the control pump 32, and less horsepower to achieve a given amountof lift.

During normal operation of the hydraulic system depicted in FIG. 6, thepistons of the slave cylinder 26 and the master cylinder 28 aresynchronized. That is, movement of the piston 120 to the left in FIG. 6causes the piston head 92 (and its piston rod 68) to move up in theslave cylinder 26 with a desired stroke length, and movement of thepiston head 92 down causes the piston 120 to return to the right. Butthis synchronous movement relies on proper amounts of control fluid inthe system, and particularly within the portion of the hydraulic circuitbetween the piston heads 92 and 122 (including chambers 96 and 128). Iftoo little control fluid is provided in this portion of the hydrauliccircuit compared to the rest of the circuit, pressure from the chambers98 and 130 (as well as the slave cylinder load and the accumulatorpressure) will cause the piston heads 92 and 122 to be too closetogether. And if too much control fluid is provided in this portion, thepressure in the chambers 96 and 128 will cause the piston heads 92 and122 to be too far apart. In either of these cases, one of the pistonheads 92 or 122 would bottom out (i.e., the piston would reach the endof its stroke) before the other has moved a desired amount. In such acondition, the slave cylinder 26 and the master cylinder 28 can beconsidered to be out-of-sync.

To facilitate proper operation, the controller 42 receives inputs fromvarious sensors and controls the pumping system components tosynchronize the cylinders 26 and 28. Sensors (e.g., proximity switches110 and 112, and linear position transducer 138) facilitatedetermination of the positions of the cylinder pistons. Based on thisinformation, the controller 42 can synchronize the cylinders 26 and 28.More specifically, at startup, the controller 42 determines whether thecylinders are synchronized. If they are not, the controller 42automatically synchronizes the cylinders before starting normaloperation of the system. That is, the controller 42 operates the pumps32 and 34, the flushing valves, or both to vary the amount of fluid inthe various chambers and, consequently, to adjust the distance betweenthe piston heads 92 and 122 in the circuit such that the pistons of bothcylinders 26 and 28 can travel their intended stroke lengths duringoperation. As one example, such synchronization can be achieved bypumping control fluid with the primary pump 32 into chamber 98 toretract the piston rod 68 and then varying the amount of fluid inchamber 128 (to properly space the piston head 122 with respect to thepiston head 92) by either pumping additional fluid into the chamber 128with the auxiliary pump 34 or by flushing fluid from chamber 128. Suchsynchronization of the cylinders 26 and 28 can also be based on otherinputs, such as feedback from other sensors 144 (e.g., pressure sensorsin the hydraulic circuit, temperature sensors, and a level sensor in thefluid tank 38 (FIG. 2)). The controller 42 could also control othercomponents of the system, such as the prime mover 36.

In some embodiments, the controller 42 provides additional controlfunctionality. For instance, the controller 42 can vary operation of thehydraulic system based on temperature detected by one or moretemperature sensors. On a cold startup, the controller 42 may operatethe system at a reduced speed (e.g., operate the primary pump at a setpercentage of maximum flow) until a desired system temperature isachieved. This may reduce cavitation of the pumps and damage to sealsand filter elements in the system. And during operation, the controller42 can cause operation of the system to be slowed if the detectedtemperature exceeds a first threshold temperature and stopped if thedetected temperature exceeds a second, higher threshold. It is notedthat continued operation at a reduced speed allows hydraulic fluid to bepassed through a cooler of the conditioning system 40, which may allowthe system to cool faster than if the system were simply stopped.

Additionally, the controller 42 may provide an emergency alarm orshut-off function to stop undesirable operation of the system, which mayinclude motion of the piston 120 outside of a desired range. By way ofexample, the master cylinder 28 and the slave cylinder 26 may beconfigured such that the maximum volume of the chamber 128 exceeds thatof chamber 96 to which it is hydraulically connected. Movement of thepiston head 122 to the left in FIG. 6 causes control fluid to flow fromthe chamber 128 to the chamber 96. But once chamber 96 is filled to itsmaximum volume (which may be determined by the controller 42 based onproximity switch 110 detecting, via trigger 114 (FIG. 5), that pistonhead 92 is at the end of the slave cylinder 26), additional movement ofthe piston head 122 in the same direction could damage the cylinders(e.g., by causing pressures within these chambers to exceed the ratedmaximum of the cylinders). In at least some embodiments, the controller42 can operate a valve to flush fluid from the portion of the hydrauliccircuit including chambers 96 and 128 if it detects (e.g., via apressure sensor) that the pressure in that portion of the hydrauliccircuit is too high.

Further, in some embodiments the controller 42 monitors the position ofthe piston 120 (via transducer 138) to facilitate synchronization andensure the piston 120 is not traveling too close to the end of themaster cylinder 28 at chamber 128. The controller 42 can be programmedwith one or more threshold distances based on the allowable maximumtravel of the piston 120. For example, if the distance from the end ofthe master cylinder 28 and the piston head 122 falls below a first, setthreshold the controller 42 can trigger an alarm to alert an operatorthat the piston 120 is moving to a distance from the end of the cylinderthat is near a minimum allowable distance. In addition to or instead oftriggering an alarm, the controller 42 can automatically activate theauxiliary pump 34 to pump additional control fluid into the chamber 128to try to push the piston 120 to the right in FIG. 6 and synchronize thesystem such that the detected distance between the piston head 122 andthe end of the master cylinder 28 is maintained above the firstthreshold. In the event the controller 42 is unable to synchronize thesystem and the distance from the end of the master cylinder 28 and thepiston head 122 falls below a second, set threshold (e.g., set to theminimum allowable distance), the controller 42 can deactivate the systemto allow servicing.

Closed-loop hydraulic systems typically have sections of dead fluid(i.e., control fluid that cannot be removed from a hydraulic circuit forcooling or filtration). Additionally, synchronization between master andslave cylinders is maintained by trying to minimize fluid loss acrossseals of the system, and dead fluid resulting from trying to minimizefluid losses can lead to damage as the fluid degrades or iscontaminated. But in some embodiments of the present technique, and asgenerally noted above, control fluid can be continually flushed from thedifferent portions of the hydraulic circuit and replaced by freshcontrol fluid. The flushed fluid can be conditioned (e.g., filtered andcooled) via conditioning system 40 and returned to the hydraulic circuitor to the fluid tank 38. During such flushing and refilling, thecontroller 42 monitors the positions of the pistons of the cylinders 26and 28 and can automatically keep or put the system back insynchronization by controlling the operation of the primary pump 32 andthe auxiliary pump 34 to replace the flushed control fluid with acorresponding amount of fresh control fluid. In some embodiments, theauxiliary pump 34 operates independently from the primary pump 32 andpulls control fluid directly from the fluid tank 38, which has fluidthat will be generally cooler and cleaner than fluid that could be drawnfrom the primary pump 32. For instance, in the embodiment depicted inFIG. 6, control fluid pumped between chamber 98 and chamber 130 tooperate the cylinders 26 and 28 can be flushed from the hydrauliccircuit and replaced with fluid drawn from the fluid tank 38 by theprimary pump 32. Further, control fluid used in the portion of thecircuit including chambers 96 and 128 can be flushed and replaced withfluid drawn from the tank 38 by the auxiliary pump 34. Control fluid inchamber 134 and the accumulators 136 can also be flushed and replacedwith fluid drawn from the tank 38 by the auxiliary pump 34.

While the aspects of the present disclosure may be susceptible tovarious modifications and alternative forms, specific embodiments havebeen shown by way of example in the drawings and have been described indetail herein. But it should be understood that the invention is notintended to be limited to the particular forms disclosed. Rather, theinvention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by thefollowing appended claims.

1. A rod-pumping system comprising: a downhole pump positioned in awell; a well string coupled to the downhole pump; a first hydraulicactuator arranged with the well string so as to enable the firsthydraulic actuator to move the well string within the well; a secondhydraulic actuator connected in fluid communication with the firsthydraulic actuator; and a control pump connected to the first hydraulicactuator and to the second hydraulic actuator in a manner that enablesthe control pump, during operation, to alternate between pumping controlfluid to the first hydraulic actuator to cause the well string to movein a first direction within the well and pumping control fluid to thesecond hydraulic actuator to cause the well string to move in anopposite, second direction within the well.
 2. The rod-pumping system ofclaim 1, wherein the control pump is connected to a rod end of the firsthydraulic actuator.
 3. The rod-pumping system of claim 1, wherein thesecond hydraulic actuator is connected to at least one accumulator setto counterbalance the full load of the first hydraulic actuator.
 4. Therod-pumping system of claim 3, wherein the connection of the controlpump to the first and second hydraulic actuators enables the controlpump to offset hydraulic balance between the first and second hydraulicactuators.
 5. The rod-pumping system of claim 1, comprising sensors tofacilitate detection of piston positions in the first and secondhydraulic actuators.
 6. The rod-pumping system of claim 5, wherein thesensors include proximity switches that facilitate detection of theposition of a piston of the first hydraulic actuator and a lineartransducer that facilitates detection of the position of a piston of thesecond hydraulic actuator.
 7. The rod-pumping system of claim 5,comprising a controller configured to receive input from the sensorsthat facilitate detection of the piston positions and to synchronize thefirst and second hydraulic cylinders.
 8. The rod-pumping system of claim1, comprising an auxiliary pump that enables the introduction of freshcontrol fluid to a hydraulic circuit that includes the first hydraulicactuator and the second hydraulic actuator.
 9. The rod-pumping system ofclaim 1, wherein the first hydraulic actuator is a double-actinghydraulic cylinder.
 10. The rod-pumping system of claim 1, wherein thefirst hydraulic actuator is coupled to a wellhead installed at the well.11. The rod-pumping system of claim 1, wherein the well string includesa sucker-rod string that is coupled to a rod clamp that engages thefirst hydraulic actuator.
 12. A rod-pumping system comprising: a slavecylinder mounted on or adjacent to wellhead equipment installed at awell; a master cylinder connected in fluid communication with the slavecylinder, wherein the master cylinder is counterbalanced with fluid fromat least one accumulator and is able to absorb the full load of theslave cylinder; and a closed-loop hydraulic circuit including a pumpconnected in fluid communication with the slave cylinder and with themaster cylinder to enable the pump to drive a piston in the slavecylinder and a piston in the master cylinder to reciprocate a wellstring within the well.
 13. The rod-pumping system of claim 12,comprising an additional pump that enables hydraulic fluid to be pumpedinto the closed-loop hydraulic circuit to replace used hydraulic fluidflushed from the closed-loop hydraulic circuit.
 14. The rod-pumpingsystem of claim 13, comprising a prime mover coupled to drive both thepump and the additional pump.
 15. The rod-pumping system of claim 12,comprising: position sensors on the slave cylinder and the mastercylinder; and a controller configured to synchronize the slave cylinderand the master cylinder based on input from the position sensors. 16.The rod-pumping system of claim 12, comprising a downhole pump coupledto the well string.
 17. A method comprising: lowering a well string thatis positioned in a well and is coupled to a downhole pump within thewell by pumping control fluid to a first linear actuator; and raisingthe well string by pumping control fluid to a second linear actuatorthat is connected to the first linear actuator.
 18. The method of claim17, wherein pumping control fluid to the first linear actuator includespumping control fluid to a rod end of the first linear actuator.
 19. Themethod of claim 17, comprising: determining the positions of pistonswithin the first linear actuator and the second linear actuator; andsynchronizing the first linear actuator and the second linear actuatorbased on the determined positions of the pistons.
 20. The method ofclaim 17, comprising alternating flow of control fluid between the firstlinear actuator and the second linear actuator by reversing flow of abidirectional pump connected to the first linear actuator and the secondlinear actuator.
 21. The method of claim 17, comprising: continuallyflushing control fluid from a hydraulic or pneumatic circuit includingthe first and second linear actuators and replacing the flushed controlfluid with fresh control fluid during operation of the first and secondlinear actuators; and maintaining synchronization between the first andsecond linear actuators during operation.